As is known, systems for harvesting energy (also known as “energy harvesting systems” or “energy scavenging systems”) from intermittent environmental energy sources (i.e., sources that supply energy in an irregular way) have aroused and continue to arouse considerable interest in a wide range of technological fields. Typically, energy harvesting systems are adapted to harvest, store, and transfer energy generated by mechanical sources to a generic load of an electrical type.
Low-frequency vibrations, such as for example mechanical vibrations of disturbance in systems with moving parts can be a valid source of energy. Mechanical energy is converted, by one or more appropriate transducers (for example, piezoelectric or electromagnetic devices) into electrical energy, which can be used for supplying an electrical load. In this way, the electrical load does not require batteries or other supply systems that are cumbersome and poorly resistant to mechanical stresses.
FIG. 1 is a schematic illustration, by means of functional blocks, of an energy harvesting system of a known type.
The energy harvesting system 1 of FIG. 1 comprises: a transducer 2, for example of an electromagnetic or piezoelectric type, subject during use to environmental mechanical vibrations and configured for converting mechanical energy into electrical energy, typically into AC voltages; a scavenging interface 4, for example comprising a diode-bridge rectifier circuit (also known as Graetz bridge), configured for receiving at input the AC signal generated by the transducer 2 and supplying at output a DC signal for charging a capacitor 5 connected to the output of the rectifier circuit 4; and a DC-DC converter 6, connected to the capacitor 5 for receiving at input the electrical energy stored by the capacitor 5 and supplying it to an electrical load 8. The capacitor 5 hence has the function of energy-storage element, energy which is made available, when required, to the electrical load 8 for operation of the latter.
The global efficiency ηTOT of the energy harvesting system 1 is given by Eq. (1) belowηTOT=ηTRANSD·ηSCAV·ηDCDC  (1)
where: ηTRANSD is the efficiency of the transducer 2, indicating the amount of energy available in the environment that has been effectively converted, by the transducer 2, into electrical energy; ηSCAV is the efficiency of the scavenging interface 4, indicating the energy consumed by the scavenging interface 4 and the coupling factor ηCOUPLE between the transducer 2 and the scavenging interface 4 (indicating the impedance matching between the between the transducer 2 and the scavenging interface 4); and ηDCDC is the efficiency of the DC-DC converter 6.
As is known, in order to supply to the load the maximum power available, the impedance of the load should be equal to that of the source. As illustrated in FIG. 2, the transducer 2 can be represented schematically, in this context, as a voltage generator 3 provided with a resistance RS of its own. The maximum power PTRANSDMAX that the transducer 2 can supply at output may be defined as:PTRANSDMAX=VTRANSD2/4RS if RLOAD=RS  (2)
where: VTRANSD is the voltage produced by the equivalent voltage generator; and RLOAD is the equivalent electrical resistance at the output of the transducer 2 (or, likewise, seen at input to the scavenging interface 4), which takes into due consideration the equivalent resistance of the scavenging interface 4, of the DC-DC converter 6, and of the load 8.
Due to the impedance mismatch (RLOAD≠RS), the power at input to the scavenging interface 4 is lower than the maximum power available PTRANSDMAX.
The power PSCAV transferred to the capacitor 5 is a fraction of the power recovered by the interface, and is given by Eq. (3) belowPSCAV=ηTRANSD·ηSCAV·PTRANSDMAX  (3)
The power required of the DC-DC converter 6 for supplying the electrical load 8 is given by the following Eq. (4)PLOAD=PDCDC·ηDCDC  (4)where PDCDC is the power received at input by the DC-DC converter 8, in this case coinciding with PSCAV, and PLOAD is the power required by the electrical load.
The efficiency of the system 1 of FIG. 1 markedly depends upon the signal generated by the transducer 2.
The efficiency drops rapidly to the zero value (i.e., the system 1 is unable to harvest environmental energy) when the amplitude of the signal of the transducer 2 (signal VTRANSD) assumes a value lower, in absolute value, than VOUT+2VTH—D, where VOUT is the voltage accumulated on the capacitor 5, and VTH—D is the threshold voltage of the diodes that form the scavenging interface 4. As a consequence of this, the maximum energy that can be stored in the capacitor 5 is limited to the value Emax=0.5·COUT·(VTRANSDMAX−2VTH—D)2. If the amplitude of the signal VTRANSD of the transducer 2 is lower than twice the threshold voltage VTH—D of the diodes of the rectifier of the scavenging interface 4 (i.e., VTRANSD<2VTH—D), then the efficiency of the system 1 is zero, the voltage accumulated on the output capacitor 5 is zero, the environmental energy is not harvested and the electrical load 8 is not supplied.